This invention relates to pressure sensitive bitumen-based adhesives which are adapted to provide increased shear strength and flow resistance at the elevated temperatures often encountered in roofing applications. The invention also relates to self-adherable waterproofing membranes comprising the adhesive compositions of this invention.
Compositions containing a mixture or blend of a bitumen, oil, and certain rubbers (usually styrene-butadiene rubbers) have been used as pressure sensitive adhesives in waterproofing applications. (For the sake of easy reference and brevity, these bitumen-oil-rubber adhesives are referred to hereinafter simply as "bitumen-based adhesives".) Generally, the bitumen-based adhesives are capable of being formed into pressure sensitive adhesive layers which can be adhered to a substrate at ambient temperature to provide a waterproofing covering. Although these layers can be self supporting and thus not require a permanently adhered support or carrier, the commerical products employing such layers generally have included a support layer to which one face of the pressure sensitive adhesive layer is adhered. Typical of the support layers which have been used are polymeric sheets, e.g. polyethylene, polypropylene, polyvinylchloride, polyvinylfluoride and polyethylene terephthalate sheets and various woven or non-woven backings. Among the more preferred support layers are the biaxially oriented or oriented and cross-laminated polyolefin films disclosed in Canadian Pat. No. 1,008,738.
Membranes comprising a waterproofing adhesive layer of a bitumen-based adhesive have been widely used in roofing applications, i.e., as self adherable roofing membranes applied to a roofdeck substrate, as below grade waterproofing coverings, e.g., in waterproofing concrete foundations, plaza decks, bridge decks, and the like, aand in pipe wrapping applications. The bitumen-based adhesive layer provides a fully adhered waterproof layer which is self sealing and provides excellent waterproofing protection, this being in addition to the waterproofing protection provided by any support layer which might be used.
Although the waterproofing roofing membranes comprising these adhesives have heretofore been capable of application to a substrate under a wide variety of conditions, it has generally been required that the temperature of the substrate to which the membrane or, more specifically, the adhesive layer is to be applied is at least about 40.degree. F., more preferably at least about 50.degree. F. This limitation is primarily due to an insufficient degree of adhesiveness or "tack" at temperatures below about 40.degree. F. Thus, application of these membranes has been restricted on a seasonal basis in the more northern climates.
The tack of bitumen-based adhesives tends to increase as the proportion of rubber is decreased or as the proportion of oil is increased. However, as a practical matter, neither of these options has proven to be a satisfactory means of improving the low temperature tack of the adhesive. Lowering the rubber content to the degree necessary to attain a significant increase in low temperature tack has resulted in an unacceptable decrease in the cohesive strength of the adhesive. This, in turn, has adversely affected the shear strength and flow resistance of the adhesive, particularly at higher temperatures of use. In addition, decreasing the rubber content has generally had an adverse affect on the flexibility of the adhesive. This can be especially troublesome at lower temperatures of use, e.g., from the standpoint of having sufficient flexibility to permit satisfactory performance while adhered to a substrate at very low temperatures, e.g., about -20.degree. F. or less, the adhesive layer having a greater tendency to crack due to embrittlement at these low temperatures.
Increasing the oil content to a degree sufficient to improve low temperature tack to a significant degree also results in unacceptable decreases in the cohesive strength of the adhesive, thus creating much the same problems at high temperatures as are encountered in lowering the rubber content.
The above considerations are especially important in the case of bitumen-based adhesives intended for use in roofing membranes as opposed, for example, to below grade waterproofing applications. In general, roofing membranes are subjected to the fullest extremes of temperature for a given climate. At the low temperatures of the roofing environment, e.g., down to &lt;20.degree. F., the maintenance of flexibility is important for minimizing embrittlement of the adhesive and resultant cracking due to foot traffic or other stresses to which the adhesive layer may be subjected. At the typical high temperatures of the rooftop environment, e.g., 140.degree. F. or higher, maintenance of high temperature flow resistance and shear strength is important for preventing flow of the adhesive e.g., on sloped roofs, and for preventing localized displacement of the adhesive, e.g., due to foot traffic or the weight of overlying ballast. Thus, the combination of low temperature flexibility and high temperature shear strength and flow resistance is particularly desirable in adhesives intended for use in roofing. In view of the adverse effect on these properties resulting from the aformentioned adjustments of oil and rubber content, such adjustments proven to be particularly unsatisfactory as a means of improving the low temperature tack of bitumen-based roofing adhesives.
Our copending application Ser. No. 724,000, filed of even date, discloses that bitumen-based adhesives comprising bitumen, oil, and an ionomeric elastomer can be formulated to provide significantly improved low temperature tack without undue adverse effect on the low temperature flexibility or high temperature shear strength or flow resistance of the adhesive. The ionomeric elastomer serves as a rubber component of these adhesives which effectively replaces part or all of the conventional rubbers (usually styrene-butadiene copolymers) heretofore used. It was found that ionomeric elastomers could be incorporated into these compositions in much lower proportional amounts than the previously used rubbers, thereby providing a significant improvement in the low temperature tack of the adhesive, and that the resultant adhesive composition could be modified, as necessary, to achieve desired low temperature flexibility or high temperature shear strength and flow resistance.
The present invention is directed to adhesive compositions of the type disclosed in the aforementioned application which are modified by the incorporation of an additional component which increases the high temperature shear strength and flow resistance to levels desired for roofing applications while maintaining the improved low temperature tack of the adhesive and its low temperature flexibility.